Replace the shadow map file inside your map with a blank one of the same demensions. It must be the same size but just blank or your map will fail to load.
For example you could make a dummy flat map of the same physical size of your map and then copy its shadow map file into your map via an MPQ editor to reset your shadows.
I advise keeping clear of the calculate shadows tool as that generates the ugly shadow maps.
So why does this happen you may be asking? Well WC3 was desiged for DirectX 7 orignally (although it was released using DirectX 8). The result is that there was no facility for realtime shadow generation during the development of the game. Shadows were still common in 3D games so to make them they decided to use a mapping approach whereby buildings and the terrain use pregenerated realistic shadows which then got mapped onto an overlay layer for the terrain. This resulted in realistic shading to the user and suited the games orignal purpose perfectly. Units on the other hand, due to rotation, could not use that sort of shading so instead they just place a blod shadow for units offset in a semi realistic way to give the appearence of some shading instead of no shading. Due to all shadows being pregenerated, the light direction had to be fixed so the model shading from the light system matched the shadow images and offsets.
The problem is that the univeral shadow map is low resolution when ingame so small terrain details can cause disproportional shadows covering a large area. Additionally the generation algerthims ignore some terrain objects and as such many objects which should cast huge terrain shadows do not. Additionally for environments with restricted light (like a dungeon), the shadows make no sense. Additionally it does not auto update cause the shadow generation is software computed (I can only guess for the slowness of it) so you can have anomilies like the screenshot where now flat terrain still has shadows from a previously existing hill until recalculation. Due to blizzard's American approach to editing they fail to provide low level interaction with the shadow map (via a brush) nor the ability to discard shadow data for when the results of auto generation are ugly.
Modern games like SC2 do not suffer from this. They use real time shadow generation (DX9+ via depth buffers and special opperations) so everything from units to terrain cast shadows on geometry at a resolution of 1024*1024 (maybe more on extreem). Even as you raise and lower terrain you can see the geometry starting to cast shadows. Additionally, due to the dynamic nature of the algerthim, you can alter the directional source of shadows as well as the shading of units. You can even change them ingame in SC2 so you can have shadows getting longer as night starts to fall until everything is in shadow.